This invention relates to solid electrolyte galvanic energy conversion cells. More particularly, this invention relates to such cells which have a capability of being operated in temperatures on the order of 2000.degree. C. without destructive thermal shock, include carbon within the cell and permit oxygen ions to pass through the electrolyte at temperatures greater than about 705.degree. C. causing formation principally of CO and secondarily of CO.sub.2. Still more particularly, this invention relates to such cells in combination with a solar energy collector for generating electricity and CO gas.
Several methods have been suggested for the direct conversion of solar energy into electricity with varying degrees of success. Two of the more efficient methods involve thermopiles and photovoltaic devices.
During the 1950's, many attempts were made to develop generators utilizing parallel and series circuits of thermocouples as thermopiles in which the hot junctions of the thermocouples were exposed to reflected and concentrated solar rays. However, conventional thermocouples provided only slight thermoelectric power. Thus, many investigations were funded to develop new thermocouples having high thermoelectric power. For example, significiant monies were expended to investigate the C-SiC thermocouple disclosed in U.S. Pat. No. 2,094,102 and in Trans. AIME, 105, 290 (1933) which had been developed by the inventor of the subject matter of this disclosure. That non-metallic thermocouple exhibits some 300 microvolts per degree centigrade and can be used continuously at 3000.degree. F. Thus, that device has about 30 times the thermoelectic power of that produced by the well known Pt-PtRh couple.
Some of the investigators during this same period determined that thermocouples which exhibited at least 1000 microvolts per degree centigrade would be required if the conversion of solar energy was to become practical by this method. See, for example, M. Telkes, J. Applied Physics, 25, 1058 (1954). To date, however, no such thermocouple has been found.
Thus, attention was directed to other methods of conversion such as photovoltaic devices. Of these, silicon and selenium cells have been the most widely utilized. However, at present, those cells are quite expensive and the power produced per cell is small (i.e., about one watt) at 2 volts per cell. These cells are, of course, tiny and the power output is related to the surface area which is exposed to solar radiation.
Very large solar cell installations are being considered in the event that the prices per cell can be reduced significantly. One projected installation at a future $1/watt cost is being planned for Arizona. This hopefully will develop 14.3 M kilowatts at a total cost of $58 billion. Also, it would require a surface area of 2200 square miles (44.times.50 miles). Thus, it has been of continuing interest in this art to give consideration to other more compact and less costly methods for directly converting solar energy into electricity.
The method and apparatus of this invention, as described below, constitutes an entirely new approach to the conversion of solar energy to electrical energy compared to those above. The apparatus and method of the invention will not only provide a large amount of electric energy, but will also simultaneously provide considerable quantities of carbon monoxide (CO) gas as a by-product. That gas can be stored and also used later as a fuel. For example, the CO may subsequently be circulated through a lower temperature cell to provide additional electrical energy by converting the CO to CO.sub.2.
The chemical reactions which are employed in the cell of the invention preferably generate more molecules of gaseous product than the number of molecules of the gaseous reactant. One such reaction known to the art is the well known "producer-gas reaction" characterized as follows: EQU C+CO.sub.2 =2 CO
that reaction was used instead of natural gas for many purposes, including open hearth steel melting furnaces prior to 1930. At present, however, it is seldomly used.
The producer gas reaction is actually a combination of two reactions which are characterized by their standard free energy (.DELTA.G.sup.o) equations, as follows: EQU 2C+O.sub.2 =2 CO, .DELTA.G.sub.1.sup.o =-53,400- 41.9T EQU c+o.sub.2 =co.sub.2,.DELTA.g.sub.2.sup.o =-94,200-0.2T , .DELTA.
by subtraction, EQU C+CO.sub.2 =2CO, .DELTA.G.sub.3.sup.o =40,800-41.7T
the high entropy term in the free energy equation, .DELTA.G.sub.1.sup.o, is responsible for the essentially exclusive production of CO above about 978.degree. K. or 705.degree. C. (1302.degree. F.) since 40,800/41.7=978.degree. K. Thus, carbon in the presence of one molecule of O.sub.2 will produce two molecules of CO above about 705.degree. C.
The relative trends of .DELTA.G.sub.1.sup.0 and .DELTA.G.sub.2.sup.0 in the above equation with temperature may be seen in FIG. 1. Because of the very small entropy term in the .DELTA.G.sub.2.sup.o equation, the standard free energy for the formation of CO.sub.2 is essentially constant over a wide range of temperatures. As shown in FIG. 1, as the temperature rises above 705.degree. C., the formation of CO is very strongly favored.
At 3000.degree. F., or 1920.degree. K., .DELTA.G.sub.1.sup.o =-133,848 calories and since .DELTA.G.sub.1.sup.0 =-4.575 T log K, log K=133,848/8784=15.238, and K=1.72.times.10.sup.15 =(pCO).sup.2 /p.sub.0.sbsb.2. With pure oxygen, pO.sub.2 =1 and with air, pO.sub.2 =0.21. The large negative free energy value and the resultant large equilibrium constant in this example together indicate the strong reaction potential to form CO at high temperatures.
The use of air instead of pure oxygen would only change this equilibrium constant to 3.6.times.10.sup.14 which is still a very large number showing that the reaction is a very strong one indeed. The fact that the term pCO is squared in the equilibrium constant and that the entropy value for the reaction is large illustrate the promise that this reaction and these observations provide at these high temperatures in connection with the invention.
Viewing the background of the invention from another standpoint, it has also long been the desire of investigators to convert the oxidation reactions of solid carbon, even as coal, directly into electrical energy by electrolytic means, as in a direct fuel cell. A fuel cell is an electrochemical device in which the chemical energy of a fuel is converted directly and usefully into low voltage direct current electrical energy. A simple exemplary prior art fuel cell is known wherein coal is supplied at the anode and air at the cathode. The coal interacts with oxide ions to form CO.sub.2 and electricity. See H. A. Liebhofsky and D. L. Douglas, "Fuel Cells" edited by G. J. Young, p. 1 (Reinhold Publishing Corp.), 1964.
However, in such a cell, most attention has been directed toward the reaction: EQU C+O.sub.2 =CO.sub.2
which has an essentially constant efficiency of nearly 100% at low temperatures i.e., below 705.degree. C. because of a very small entropy term in the free energy equation at lower temperatures for the reaction, as discussed above. See, W. T. Grubbs and L. W. Niedrach, "Fuel Cells," Direct Energy Conversion, Symposium, McGraw-Hill, 1966, p. 45.
However, it has generally been mistakenly assumed that this advantageous free energy situation will project to 2000.degree. C. Instead, at temperatures above 705.degree. C., the reaction: EQU 2C+O.sub.2 =2CO
occurs to produce CO in preference to the formation of CO.sub.2, as is illustrated in FIG. 1. Thus, this reaction which predominately produces CO is the one which must principally be considered at temperatures above 705.degree. C.
One so-called Redox fuel cell used CO as a fuel at about 800.degree. C. in which the CO.sub.2 which was formed was passed over a hot bed of coal to be converted into CO and passed back through the cell for electrochemical oxidation. See, The Encyclopedia of Electrochemistry, Reinhold Publishing Corp., p. 619, (1964).
In considering the physical implementation of the promise of the foregoing observations, use of solid electrolytes was considered. Starting with the work of Kiukkola and Wagner, J. Electrochemical Soc. 104, 379, (1957), solid electrolytes such as calcia-stabilized zirconia have become widely used for the solution of many different types of problems of commercial interest. See. U.S. Pat. Nos. 3,619,318; 3,752,753; and 3,773,641 to G. R. Fitterer.
This material is available either in the form of a closed-end tube composed completely of the electrolyte or in the form of a disc or a pellet of calcia-stabilized zirconia supported in the end of an insulating refractory tube. It is conceivable that the reaction described herein may be operated with various solid electrolytes providing that they are permeable to oxygen ions at the temperatures of interest. Further, such cells may be miniaturized so that many of such cells may be heated to very high temperatures with the same solar reflector system.
Calcia-stabilized zirconia has the unique property of permitting the passage of only oxygen ions through its walls because of the presence of oxygen vacancies in its atomic structure. Thus, a difference of oxygen pressure on either side of the electrolyte will cause an EMF to be established.
Thus, the prior art efforts suffered from a number of deficiencies which are sought to be overcome by this invention. Accordingly, it is a broad object of this invention to provide a solid electrolyte fuel cell capable of operating at temperature at least as high as 705.degree. C. and as high as 2000.degree. C. to utilize the favorable high temperature reaction of carbon to produce carbon monoxide.
It is a further object of this invention to produce a fuel cell as described above in an embodiment which uses concentrated solar energy as a source of heat to permit the cell to produce CO and electricity.
It is another object of this invention to produce directly a considerable amount of electrical energy and CO gas which may either be burned for additional energy or directed to a second solid electrolyte fuel cell to generate additional electricity and CO.sub.2 as an acceptable by-product.
These and other objects of this invention will become apparent from a review of the foregoing background of the invention and the following written description of the invention taken in conjunction with the accompanying drawings.